1. Field of the Invention
This invention relates to the medical instrument field and more particularly to catheters and probes which cause the freezing of tissue cells.
2. Related Art
Cryogenic devices for surgical applications have been described in the patent literature. Generally, the cold tissue is at least below -60.degree. C. and often below cryogenic temperature (below -150.degree. C.). For instance, A. S. J. Lee (U.S. Pat. No. 3,298,371) describes a liquid nitrogen freezing probe that can be used for stereotaxic neurosurgery in which selected tissues are deadened. In other devices, the use of Joule-Thomson valves to obtain cryogenic cooling in cryosurgical instruments is empoyed (for instance, U.S. Pat. Nos. 3,502,081 and 3,696,813). Later developments, which are aimed at resolving problems of reproducibility of necrosis (the deadening of cells) and release of the frozen tip from the frozen tissues by rewarming the cryogenic tip, are described in U.S. Pat. No. 3,782,386, its reissue U.S. Pat. No. Re 28,657, and U.S. Pat. Nos. 3,913,581 and 4,202,336.
The mechanism involved in cryogenic cell deadening is believed to involve the freezing of the internal cellular matter, expansion of the frozen cellular matter and the consequent rupture of the cell's membranes. The prior art indicates that cryogenic cell deadening requires very rapid supercooling of the ablated tissues. According to Robert K. Mitchiner (U.S. Pat. No. 4,275,734, which is incorporated herein by reference) cooling rates of 5.degree. C./sec or less cause unpredictable necrosis, and one needs to achieve cooling rates in excess of 10.degree. C./sec to achieve consistent tissue necrosis.
Nir Merry, et al. (U.S. Pat. No. 4,946,460) uses very slow cooling rates (few degrees/minutes) which cause partial necrosis. Merry's devices are aimed at the eradication of malignancies, and he tries to destroy the vasculature permeating the target lesion. The malignancy is destroyed, when its blood vessels are destroyed, due to blood supply deprivation. Merry, et al. states that at slow cooling rates ice forms in the blood vessels, rejecting solutes (mostly salts) to the yet unfrozen blood and the freezing temperature of the concentrated saline drops. Water from neighboring unfrozen tissues migrates through the blood vessels' membranes (due to the high osmotic pressure in the now concentrated solution) until the expansion of these vessels leads to their destruction. Yet other researchers (Marchenko, et al. U.S. Pat. No. 4,528,979) first freeze the target tissues with a cryodestructor which achieves only partial necrosis, and then applies an ultrasonic beam to the frozen tissue, which completes the tissue cells' destruction.
Most current cryosurgical probes cool using very high pressure gas which undergoes expansion through a Joule-Thomson valve. Such probes require walls that can withstand the high gas pressures and such thick walls make the devices bulky and impossible to use in a number of procedures. When a combination of the Joule-Thomson effect, together with the use of evaporation cooling is used, similarly high gas pressures are used and the devices are also bulky. All these above-mentioned cryogenic devices use relatively thick walls, so that the probes are relatively inflexible. When evaporative cooling with liquid nitrogen is used (for instance, in Marchenko's devices) a flexible catheter is not possible as elastic materials become embrittled at the applied cryogenic temperatures. There are a number of procedures, particularly treatment of severe arrythmia, where the availability of a flexible freeze ablation catheter allows for non-surgical intervention. This cannot be achieved with cryosurgical probes of the prior art due to their inflexibility.
In addition, it is often desired to keep the tip of the cryosurgical probe at ambient temperature, or at least above freezing, until actual cryogenic necrosis is desired. This is difficult because the cooling rates reached are often too slow to achieve the desired reproducible necrosis. To overcome this shortcoming, Mitchiner U.S. Pat. No. 4,275,734 controls the temperature of the cryogenic gas supply cylinder and raises its temperature somewhat above ambient. As a result, the pressure of the cryogenic gases is maintained within the range of 600 to 850 psi., which higher than in most cryosurgical devices. Such high pressures require probes that can withstand these pressures, and systems to operate these probes that are costly and difficult to operate.
In general, the prior art cryosurgical devices could not provide for flexible, catheter-like cryogenic probes that are able to be directed to specific pathological locales, either through the arterial system or through a tubular conduit inserted in small incisions.